Low Temperature Poly-Silicon (LTPS for short) thin film transistors have higher electron mobility compared with conventional amorphous silicon thin film transistors. The electron mobility of a LTPS thin film transistor can generally reach 50-200 cm2/V-sec, and thus the area of channel can be effectively reduced, so that the area of the thin film transistor can be reduced. By applying the LTPS thin film transistors in a display device, the aperture ratio and integration of the display device can be improved, and as a result, the brightness of the display device can be improved while the power consumption thereof is reduced.
LTPS is generally used for forming an active layer of a thin film transistor. At present, preparation process for an active layer includes steps of: depositing amorphous silicon (a-Si) first; then converting the amorphous silicon into poly-silicon (P-Si) by Excimer Laser Annealing (ELA for short); and subsequently, adding dopant ions by ion implantation to adjust the threshold voltage (Vth) of the thin film transistor. In the process of adding dopant ions by ion implantation, the interface of the poly-silicon is inevitably damaged by high-energy ions, and can only be repaired incompletely even after subsequent high-temperature annealing, which deteriorates stability of the thin film transistor; meanwhile, the energy of ion implantation itself has a certain distribution, in the subsequent activation process, the doped ions can hardly be distributed evenly and bonded tightly with the neighboring atoms, which makes the thin film transistor even more unstable under the impact of external energy such as heat, light and electricity.
Research results indicate that, interface characteristics have a remarkable effect on the overall characteristics of the thin film transistor, and therefore, the interface needs to have relatively high stability and other excellent properties. Since the interface and crystalline portion of poly-silicon may be damaged and have a relatively low stability, the stability and other properties of the thin film transistor are impacted. Research results further indicate that the main reason why the thin film transistor is unstable lies in the defect states, dangling bonds, movable ions, and the like near the interface, and the defect states at the P-Si side have a great impact on the stability of the device.